This chapter presents the design, fabrication and performance of a roof-mounted compound parabolic concentrators (CPC) for electricity generation. A truncated asymmetric compound parabolic concentrator with a concentration ratio of 1.83, acceptance -half angles of 60° and an absorber width of 125 mm has been designed, constructed and experimentally characterized on the roof of the Simon Perry Building at Trinity College Dublin, Ireland (53.344295, -6.252416). A CPC and a PV system were tested under outdoor conditions with 4-cell PV strings. The first challenge was to determine the power generation, efficiency and temperature of both systems and compare the results. The experiment revealed that at solar incident radiation of 800 W/m2, the CPC system increased the power and efficiency by 34% (power by factor 1.34) and 22% (efficiency by factor 1.22), respectively, compared with PV system. However, the solar cell temperature in the CPC system increased by 74% (temperature by factor 1.74). In order to analyse the temperature, a convective cooling CPC system was tested. The result showed that when the temperature increased by 22% (temperature by factor 1.22) compared with the PV system, the power and efficiency improved by 84% and 65%, respectively.

Anti StokesRaman scattering

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Most light passing through a transparent substance undergoes Rayleigh scattering. This is an elastic effect, which means that the light does not gain or lose energy during the scattering. Therefore it stays at the same wavelength. The amount of scattering is strongly dependent on the wavelength, being proportional to λ-4. (It is this fact that makes the sky blue, the shorter wavelength blue components in the Sun’s light are Rayleigh scattered in the atmosphere far more than the longer wavelengths. Blue light is then seen coming from all over the sky. The scattering of blue light from its direct path from the Sun also causes the Sun itself to appear yellow.)

Raman scatteringvs Rayleighscattering

McCormack S (2016) PV systems & solar thermal systems. In: J2: solar energy conversion and applications lecture notes. Trinity College Dublin, Dublin

Raman scattering (sometimes called the Raman effect) is named after Indian physicist C. V. Raman who discovered it in 1928, though predictions had been made of such an inelastic scattering of light as far back as 1922. The importance of this discovery was recognised even then, and for his observation of this effect Raman was awarded the 1930 Nobel Prize in Physics. This was and remains the shortest time from a discovery to awarding of the Prize. In fact Raman was so confident that he arranged his travel to Stockholm several months in advance of the recipients being announced! This confidence seems quite justified, given that within a year and a half of his discovery, more than 150 papers mentioning the effect had been published. Since then Raman scattering has given rise to a number of important technologies, and foremost among these is Raman spectroscopy.

StimulatedRaman scattering

In Rayleigh scattering a photon interacts with a molecule, polarising the electron cloud and raising it to a “virtual” energy state. This is extremely short lived (on the order of 10-14 seconds) and the molecule soon drops back down to its ground state, releasing a photon. This can be released in any direction, resulting in scattering. However since the molecule is dropping back to the same state it started in, the energy released in the photon must be the same as the energy from the initial photon. Therefore the scattered light has the same wavelength.

Raman scatteringpdf

Ortega, A., Chandra, S., McCormack, S.J. (2022). Design and Characterization of a Roof-Mounted Compound Parabolic Concentrator. In: Sayigh, A. (eds) Sustainable Energy Development and Innovation. Innovative Renewable Energy. Springer, Cham. https://doi.org/10.1007/978-3-030-76221-6_98

The shift due to the Raman effect is determined by the spacing between the vibrational states and the ground states i.e. by the phonons of the system. The Stokes and anti-Stokes scattered light will be shifted an equal distance on opposite sides of the Rayleigh scattered light. Therefore the spectrum is symmetrical about the wavelength of light used, apart from the difference in intensities.

Mallick T (2003) Optics and heat transfer for asymmetric compound parabolic photovoltaic concentrator for building integrated photovoltaics. PhD thesis. University of Ulster, Newtownabbey, N.I.U.K

Only about 1 in 107 photons undergo Stokes Raman scattering and so this is usually swamped by the far more prominent Rayleigh scattering. The amount of anti-Stokes scattering is even less than this. Click here for an explanation.

Solar Energy Applications Group, Department of Civil, Structural and Environmental Engineering, School of Engineering, Trinity College, Dublin, Ireland

The authors would like to acknowledge the European Research Council grant PEDAL: (639760), H2020 IDEAS (815271) and support funding from Science Foundation Ireland (SFI) and insights from the PEARL PV COST Action Network.

Ramaneffect

In one of Raman’s experiments demonstrating inelastic scattering he used light from the Sun focused using a telescope to obtain a high intensity light. This was passed through a monochromatic filter, and then through a variety of liquids where it underwent scattering. After passing through these he observed it with a crossed filter that blocked the monochromatic light. Some light was seen passing through this filter, which showed that its wavelength had been changed.

Ortega A (2017) Design and characterization of a roof-mounted compound parabolic concentrator with luminescent down sifting layers. Msc Thesis. Trinity College Dublin, Dublin, Ireland

Raman scattering is different in that it is inelastic. The light photons lose or gain energy during the scattering process, and therefore increase or decrease in wavelength respectively. If the molecule is promoted from a ground to a virtual state and then drops back down to a (higher energy) vibrational state then the scattered photon has less energy than the incident photon, and therefore a longer wavelength. This is called Stokes scattering. If the molecule is in a vibrational state to begin with and after scattering is in its ground state then the scattered photon has more energy, and therefore a shorter wavelength. This is called anti-Stokes scattering.